A thirteen-year-long broadband view of BL Lac

Abstract

We report optical, ultraviolet, X-ray and 𝛾-ray observations of BL Lac objects prototype BL Lacertae carried out over a period of nearly 13 years, between 2008 and 2021. The source is characterized by strongly variable emission at all frequencies, often accompanied by spectral changes. In the 𝛾-ray band several prominent flares have been detected, the largest one reaching the flux of 𝐹 𝛾 (> 196.7 MeV) = (4.39 ± 1.01) × 10 −6 photon cm −2 s −1 , corresponding to the isotropic luminosity of 10 47 erg s −1 . At the time of the first major X-ray flare, on MJD 56268.65, a traditional harder-when-brighter trend was observed. During the brightest flare, which occurred on MJD 59128.18, the source instead exhibited a softer-when-brighter trend due to a shift of the synchrotron peak to 10 16 Hz, well into the HBL domain. The widely changing multi-wavelength emission of BL Lacertae was investigated by fitting one-zone and two-zone leptonic models to 520 high-quality and quasi-simultaneous broad-band spectral energy distributions. The HBL behaviour observed during the brightest X-ray flare is interpreted as due to the emergence of synchrotron emission from freshly accelerated particles in a second emission zone.

Flaring activity from magnetic reconnection in BL Lacertae

“…We also found hints of a shorter variability timescale of 2.4 ± 0.9 min (2.6𝜎, post-trial) in 30 s binned Swift-XRT data. Similar results are reported by D'Ammando (2021); Sahakyan & Giommi (2021). A sub-hour variability of 46 ± 24 min is observed on MJD 59331 during the brightest 𝛾-ray state of the source.…”

The first hard X-ray spectral catalogue of Blazars observed by NuSTAR

“…16), with a typically hard and variable X-ray spectrum, but also large soft X-ray flares (the largest one recorded in late 2020) during which the X-ray slope becomes much steeper, likely as the result of the synchrotron peak shifting to higher energies, turning this source from an IBL into a HBL blazar. The HBL component was interpreted as due to the emergence of synchrotron emission from freshly accelerated particles in a second emission zone (Sahakyan & Giommi 2021). NuSTAR observed BL Lac three times (Rani et al 2017), in all cases detecting a hard X-ray spectrum, with a further flattening above 10 keV in at least two occasions (magenta points in Fig.…”

“…16), probably indicating the on-set of a second, much harder component, detected also during the faintest state, independent of the softer (∼ 1-10 keV) X-ray emission and possibly related to the 𝛾-ray emission. Detailed modeling of BL Lacertae SEDs between 2008 and 2021, including the periods of NuSTAR observations, is presented in Sahakyan & Giommi (2021).…”

“…the VOU_Blazar tool and the SSDC SED builder, can be combined to build timedependent SEDs and can be used for various purposes, including detailed physical model fitting. One such example is the recent work of Sahakyan & Giommi (2021), which fitted blazar models to 13 years of broad-band multi-frequency observations of BL Lacertae, the prototype of the class of blazars.…”

Astrophysical Neutrinos and Blazars

“…IBL blazars also show large ν S peak changes which cause them to occasionally become HBLs during high states. One such example is shown on the left side of Figure 3 for the case of BL Lac, which shifted ν S peak from typical values around 10 14 Hz to ∼ 10 16 Hz during a large flare [59,60]. Several other IBL/HBL transitional sources are known; some of these, e.g., OJ 287, TXS 0506+056, PKS 2005−489, PG 1553+113, are among the blazars that have been frequently observed by Swift, and their SEDs can be found in Ref.…”